1. Field of the Invention
This invention relates to a digital-signal reproducing apparatus, and more particularly, to an apparatus for reproducing digital signals from a recording medium in which n-channel (n being an integer greater than or equal to 2) digital signals are recorded on a plurality of parallel tracks.
2. Description of the Prior Art
An example of a multitrack digital-signal reproducing apparatus is a digital video tape recorder (termed hereinafter DVTR). In the present specification, an explanation will be provided illustrating the DVTR.
In general, video signals have a wide band. Hence, the amount of data per unit time of the digital video signals in which video signals are digitized is large, and it is difficult to serially perform magnetic recording and reproduction of the digital video signals. Consequently, it has been considered to provide such digital video signals in multiple channels, and decrease the data rate of each channel. In a DVTR, it is usual to perform multichannel recording and reproduction.
When the entire digital signal processing in a DVTR is performed in time series, a part of an image is completely lost due to the generation of burst, errors caused by damage in a tape, loading of a magnetic head and the like. Therefore, it becomes difficult to perform correction by interpolation and the like, causing an unfavorable result. Furthermore, since the majority of a data matrix (ECC block) to which an error correcting code (ECC) is added is erroneous, the entire ECC block provides erroneous data even if a code having a high error correction capability is added, and so it is very ineffective.
In general, therefore, in a DVTR, recording and reproduction are performed under a state in which an ECC block is divided into plural sections which are dispersed relative to time series. At this time, it is also devised such that orders of data of each picture element on a picture surface are changed in time series, and an excellent interpolation can be performed even when error correction is impossible. In such a DVTR, processings, such as encoding and decoding of an ECC, arrangement and conversion of data and the like, are performed so as to be completed making video signals for a predetermined time interval a unit. In a DVTR in which multichannel recording and reproduction are performed, the above-described processings are completed forming video signals recorded on tracks the number of which is an integer multiple of the number n of multiple channels into a unit. This prevents indefinite signal processing while reproducing, In a DVTR, it is typical to set the rotation frequency of rotating heads to an integer ratio relative to the frame frequency of the video signals. This is advantageous in that the configuration of servo-system circuits, processing circuits of video signals and the like are simplified.
Accordingly, in a multichannel DVTR, in general, video signals for one frame are recorded on (n.times.j) tracks (n is the number of multiple channels, and j is an integer no smaller than 1), and the above-described processings are completed forming video signals recorded on (n.times.i) tracks (i is an integer no smaller than 1) into a unit.
FIG. 1 is a diagram showing a head configuration of a multitrack DVTR. In FIG. 1, eight rotating heads HI-H8 are disposed on a rotating drum 1. The rotating heads H1-H4, and the rotating heads H5-H8 are adjacently disposed, respectively. The heads H1-H4 are also disposed so that they simultaneously trace on a magnetic tape. Heads H5-H8 are similarly disposed to simultaneously trace a magnetic tape. The rotating heads H5-H8 are rotated having a 180.degree. phase difference relative to the rotating heads H1-H4. The heads HI-H4 and the heads H5-H8 alternately trace a magnetic tape which is wound around the drum 1 over an angle range of no smaller than 180.degree. to perform four-channel recording.
FIG. 2 shows a track pattern recorded on a magnetic tape T by the heads shown in FIG. 1. Tracks indicated by Tr1 -Tr8 show tracks recorded by the heads H1-H8, respectively. By tracing the heads H1-H4 from positions shown by reference characters H1-H4 in an obliquely upward direction in FIG. 2, four-channel recording is performed while forming the tracks Tr1-Tr4. Four-channel reproduction is also performed by tracing the tracks Tr1-Tr4. The heads HI, H3, H5 and H? have an identical azimuth angle, and the heads H2, H4, H6 and H8 also have an identical azimuth angle which is different from that of the heads HI, H3, H5 and H7. A so-called azimuth recording is thereby performed.
In the DVTR of the present example, the number of rotations of the drum 1 is 1800 rpm, and the video signals for one frame are recorded on eight tracks. The above-described signal processing is completed for the eight tracks, that is, the video signals for one frame. Since the frame frequency thereby coincides with the rotation frequency of the drum 1, and signal processing is performed for one-frame unit, it is possible to commonly control the timing of each unit, and thus a DVTR having a simple circuit configuration can be realized.
In the above-illustrated DVTR, signal processing while reproduction is performed forms reproduced signals of the heads H1-H4 and succeeding reproduced signals of the heads H5-H8 into a unit. Accordingly, the heads H1-H8 necessarily trace the tracks TP1-Tr8 respectively, while reproducing. Although it is possible to reproduce signals even when, for example, the heads H1-H4 are tracing the tracks Tr3-Tr6 having an identical azimuth angle (shown by HI'-ZH4' in FIG. 2), or the tracks Tr5-Tr8 (shown by HI"-H4" in FIG. 2), it is impossible to return to original video signals since signal processing is performed, as described above, forming reproduced signals of the heads H1 -H4 and succeeding reproduced signals of the heads H5-H8 into a unit.
Hence, in this kind of DVTR, tracking control has heretofore been performed so that the head H1 necessarily traces the track Tr1. However, since a track as a control target for the head H1 exists only once per eight tracks, a tracking error of .+-.4 tracks is generated. Consequently, when tracking control is disrupted immediately after starting of the apparatus or for some other reason, a very long period is required before a tracking-control draw-in state is obtained. During this period, reproduced video signals cannot be obtained. Accordingly, video signals are not reproduced for a long period at the start of an apparatus or when tracking is disrupted, and so a reproduced picture is very unsightly.
Furthermore, a pattern of recorded signals for tracking control must be of an eight-track period, and so circuits for tracking in both recording and reproducing systems inevitably become complicated. In addition, in an apparatus in which a pilot signal is recorded on a part of a track, and tracking control is performed using the pilot signal, not only does the circuit become complicated, but also the pilot signal requires a large recording area. These facts hinder a high-density recording.
Such problems increase in accordance with an increase in the number of multiple channels and an increase in the number of tracks in which signal processing is completed. These will become greater obstacles when an attempt is further made to perform high-density recording of wide-band signals.